The present invention relates to a light control element, an more particularly to an all-optical light control element from which an optical signal can be output under the control of light.
The present invention also relates to an organic thin-film element which can control signals with electricity or light, store information and transmit information.
An advanced information-oriented society will emerge in the twenty-first century. It will then be necessary to transmit and process super mass storage information at high speeds (e.g., terabits or more per second), with high precision and high efficiency. Even today such a tremendous amount of information can be transmitted through trunk lines and processed by the data-processing apparatus on the trunk lines. It is expected, however, that such super mass storage information will need to be transmitted and processed also on subscriber sides.
The data-processing speed of the electronic system widely used now is limited, about 50 gigabits per second at most. The system cannot transmit or process such super mass storage information. In view of this, an optical system capable of transmitting and processing information at much higher speeds than the electronic system now available will become desirable in the near future.
In the optical system, important is an light control element which performs modulation and demodulation at an ultrahigh speed, i.e., an all-optical light-control element. The optical, light-control optical element has no limitation to a working band due to the CR product in an electronic circuit. It can effect an ultrahigh speed control of modulation and demodulation. The optical light-control element is advantageous in that its output light can be used as control light or signal light without any modifications.
In an optical system with optical light-control elements used as an optical gate element, an optical logic element, an optical bistable element, an optical pulse-controlled element and the like, information can be transmitted and processed at an ultrahigh speed, unlike in the electronic system. The optical system can be an optical computer which is free from low-speed transmission and processing of information inherent in the electronic system.
In such control of light signal using light, a nonlinear optical effect plays an important role.
Nonlinear optical effects of third order, i.e., changes in refractive index and absorption coefficient, depending on an optical intensity, draw attention as a fundamental principle in optical light-control element.
The refractive index n and the absorption coefficient .alpha. have the following relations with the light intensity I in the nonlinear optical effects of third order: EQU n=n.sub.0 +n.sub.2 .times.I EQU .alpha.=.alpha..sub.0 +.alpha..sub.2 .times.I
wherein n.sub.0, n.sub.2, .alpha..sub.0 and .alpha..sub.2 are, respectively, a linear refractive index, a non-linear refractive index, a linear absorption coefficient and a non-linear absorption coefficient.
As is clear from the equations, n.sub.2 and .alpha. must have a higher non-linearity in order to obtain a larger change in an optical characteristic with light of a low intensity. The non-linearity of the nonlinear refractive index or the nonlinear absorption coefficient determines a non-linear susceptibilty of the third order .chi..sup.(3).
Investigations on material showing non-linear optical effects of third order have advanced in three material systems, a semiconductor ultra fine particulate system, a semiconductor superlattice system and an organic .pi. electron system material.
If direct transition semiconductor is used as a semiconductor superlattice based material, it will have a large nonlinear optical response due to a band filling effect. When GaAs/AlAs is used as a direct semiconductor, a non-linear susceptibility of third order .chi..sup.(3) as high as 5.times.10.sup.-2 esu in a MQW is obtainable.
A semiconductor superlattice based material can have such a extremely large susceptibility .chi..sup.(3). However, the response speed is low. This is because the response speed depends on the recombination life time of optically excited carriers, where is the life time of an excited carrier is on the order of nanosecond. Therefore, an attempt has been made to increase surface recombination speed in order to increase the response speed. It is still impossible to achieve a high-speed response, especially a repetitive response of high speed at p sec level.
A semiconductor material, composed of five particles may provide a response speed as high as tens of p sec, with an increase in a surface recombination speed, if the particles have diameters of 10 nm or less. However, as eperiments with doped glass CdS.sub.x Se and the like show, a nonlinear susceptibility of third order .chi..sup.(3) is on the order of 10.sup.-9 to 10.sup.-8 esu, and a higher nonlinear susceptibility of third order has not been obtained.
An organic .pi. electron system material achieves a high response speed on the order of f sec, since a non-linearity response is caused by pure polarization of .pi. electrons. However, even a polydiactylene based para-toluene sulfonic acid ester derivative, which has been recognized to have the highest susceptibility of third order, a non linear susceptibility .chi..sup.(3) of 8.times.10.sup.-10 esu, which is still very small.
A light control element using .pi. electron system material is known. Since nonlinearity derives from pure polarization of .pi. electrons, many attempts have been made to increase a conjugated length of .pi. electrons by using an unidimensional .pi. conjugate based highpolymer such as polydiacetylene, transpolyacetylene or polyallylenevinylene, or a cyclic .pi. conjugate compound such as phthalocyanine so that a three-dimensional nonlinear susceptibility of third order .chi..sup.(3) may be increased. However, such approaches can hardly attain a sharp increase in non-linearity.
Besides, a light control element using .pi. electron system material has to be irradiated with extremely intense light to cause a desired optical change. This is inevitably because an organic .pi. electron system material has such a low susceptibility of third order, as mentioned above. If the element is irradiated with high-intensity light, the material will have not only optical damages but also thermal damages and, will have a new thermal effect.
Under such circumstances, a light control element has been desired which has not only a high susceptibility of third order but also a high response speed.
Recently, molecular electronics attracts a growing attention. In this field of art, efforts are made to develop a device having a new function, unknown to a traditional device, taking advantage of physical properties peculiar to an organic crystal or an organic molecule. Active researches have been so far conducted on applications of an organic molecule to a nonlinear optical element of second order, an electric switching element, an injection-type light emitting element, a solar cell, an optical information recording medium and the like.
These researches are to improve a device in the terms of characteristics and production cost, by using an organic material having physical properties exploited in the device made of an inorganic material system. One example of physical phenomena inherent in an organic molecular system, which also draws attention, is charge transfer observed in a kind of organic complex crystal.
Among organic materials are a donor molecule which has a small ionization energy and is easy to become a cation by giving away electrons to another molecule, and an acceptor molecule which has an electron affinity and is easy to become an anion when receiving electrons from another molecule. As well known in the art, a compound called a charge transfer complex is formed between these two kinds of molecules. For example, a compound formed between perylene and tetracyanoquinodimethane (TCNQ) is composed of neutral compounds. On the other hand, a compound formed between tetramethylphenylenediamine (TMPD) and TCNQ is one in which individual compounds are respectively ionized positive and negative. It is also known that neutral-ionic transition is observed in a compound between tetrathiafulvarene (TTF) and chloranil with change in temperature or pressure (J. B. Torrance et al.: Phys. Rev. Lett., 46, 253 (1981)).
Such a charge transfer phenomenon of an organic molecule may be utilized as a working principle of an electric element and an optical element. In this case, it is important how to cause charge transfer by an electric field or light with good efficiency and good controllability. Recently, an interesting result has been reported in regard to electrical characteristics of charge transfer complexes (Yoshinori Tokura et al. preliminary reports for the 1988 autumn meeting held by the Japanese society of physics, 3a-S4-1, 3a-S4-2, 3a-S4-3 and others; Y. Tokura et al.: Physica, 143B, 527 (1986)). The reports disclose that a mixed stack complex crystal having an alternate laminar structure, in which donor molecules and acceptor molecules are stacked with molecular planes facing each other, exhibits a high anisotropy in dielectric constant, a dielectric constant of 100 to 1000 in the direction of stacking, and nonlinear electric conduction and switching characteristic observed under application of an electric field of the order of 10.sup.3 to 10.sup.4 /cm. As one of the suspected reasons is that an ionic domain in a neutral crystal or a neutral domain in an ionic crystal, both thermally or electrically generated in the bulk, is dynamically moved under the influence of an electric field.
This phenomenon is related with the neural-ionic transition and is a very localized change. The whole crystal does not change macroscopically. In the current state of the art, a macroscopic neutral-ionic transition is not produced by application of an electric field or radiation of light.
The electric field must be aligned with the direction of a stacking axis of donor molecules and acceptor molecules in order to cause macroscopic neutral-ionic transition with an electric field. To provide a device in which characteristics of an organic molecule are utilized, it is necessary to control not only the thickness and structural homogeneity of the film but also the positions of individual molecules, the arrangement of adjoining molecules and the stacking orientation of molecules.
Recently a Langmuir-Blodgett (LB) method has been drawn attention as a method of fabricating an ultra thin film in which molecular orientation and configuration are controlled. This method is to fabricate super lattice films made of the same kind or different kinds of molecules, by stacking unimolecular films formed on a water surface one by one. However, a packing condition or uniformity in configuration of molecules in a film extended over a water surface is poor, and an unimolecular film structure is sometimes disturbed during stacking the film on a substrate. Accordingly, a related technology has not reached such a level as to fabricate a superlattice thin film in which molecules are controlled in orientation across the entire film or between adjoining stacked layers. To improve a film forming technique by a LB method, molecules suitable for the LB method and a should be designed, and method of synthesizing such molecules should be decrized.
On the other hand, a vacuum evaporation method has been studied as a technique which can be applied with ease to an organic molecule of almost all kinds without requiring a special molecular design. In the vacuum evaporation method, however, an evaporation source of molecules is gasified and gasified molecules then cohere. Therefore, the film structure and the molecular orientation in the film may vary in accordance with a balance among the supply speed of gasified molecules, the surface diffusion of a molecule adsorbed on the surface of a substrate and the crystallization speed, and also with an interaction between an adsorbed molecule and a substrate surface.
Studies on an organic film heretofore have been mainly directed to a process of growing a thin film on various substrates and to the orientation of molecules in the thin film of a long chain hydrocarbon based linear molecule or a planar molecule such as phthalocyanine. Used as substrates are an alkali halide single crystal and a metal single crystal for evaluation by observation with an electron microscope or an electron diffraction technique, quartz for an optical evaluation and an Si single crystal for an electrical evaluation. As conditions of evaporation, the influences of the temperature and evaporation speed have been investigated. Vincett et al. of the United States of America have reported that a uniform, continuous film can be formed by setting a substrate at a temperature value about one third of the boiling point of an evaporation material, expressed in absolute value, regardless of kinds of substrates. However, simple optimization of evaporation conditions is not enough to control molecular orientation in an organic thin film on any kind of substrate surface.
Studies have been conducted on the influence of a substrate on molecular orientation in an organic film vapor-deposited on the substrate.
(1) Karl of West Germany has reported that a thin vapor-deposited film of several molecular layers of perylene tetracarboxylicdianhydride formed on a clean Si single crystal surface has molecular planes oriented parallel to the surface of the substrate.
(2) Hara has reported that in his study on a phthalocyanine vapor-deposited film prepared by means of a molecular beam evaporation method, a uniform, continuous film are obtained with its molecular planes oriented in parallel with the surface of substrate in conditions of ultra high vacuum and an evaporation speed as low as of the order of 0.1 nm/min, while only a discontinuous film is formed in a high vacuum evaporation under ordinary conditions. In this study, as a substrate, MoS.sub.2 of a layered compound has been used in light of the concept of van der Waal's epitaxy in order to avoid a lattice mismatch with an organic crystal.
(3) Harada et al. have reported that, in a vapor-deposited film of pentacene having several molecular layers formed on a graphite substrate, molecular planes are oriented in parallel with the surface of the substrate.
Though many studies have been done in a variety of aspects concerning thin film formation by means of a vacuum evaporation method, a standardized understanding has not yet been established about a film structure and control factors in molecular orientation.
The inventors hereof have carried out an intensive research on the above-mentioned issues, paying attention to an interaction between a substrate and a vapor-deposited molecule thereon. They have found that a uniform, continuous film can be formed, if conditions of vapor-deposition are properly adjusted by the use of a highly oriented graphite substrate. They assume that orientation effect on the highly oriented graphite substrate is based on a dispersion force between the substrate and .pi. electron system compound molecules.
On the basis of such an assumption, the inventors have tried to improve a method of forming a molecular orientation control layer on a substrate, by using a thin film having a skelton analogous to graphite, and a method of adjusting a polarizability of surface atoms on the substrate in consideration of a dispersion force dependent on an electronic polarizability.
However, it is actually difficult to form such a molecular orientation control layer on any kind of a substrate. There exists a great obstacle against a development of an organic thin film element using an oriented crystal thin film.
As described above, to optimize functions characteristic of an organic thin film, it is necessary to form a thin film structure in which the molecular orientation is controlled with respect to the surface of a substrate or in which a crystallographic axis orientation is controlled in the thin film.
In order to render a control on molecular orientation practical, however, is a technology is required which can prepare not only a flat outermost surface in a molecular scale regardless of original surface irregularity of the substrate but also a good outermost surface with high controllability on molecular orientation.